The invention relates to a catalyst which consists of amorphous carbon with molecular planes that have curved surfaces and contain six-membered and non-six-membered carbon rings, optionally having at least one catalytically active, low-valency metal covalently bound thereto. The invention also relates to methods of producing the catalyst and applications thereof.
In many applications involving the use of catalytically active metals on a support system, it is of advantage if the latter is a carbon-based system. However, the use of graphitic material as support has the disadvantage that, due to the graphite""s planar layering, there is only little interaction with the metals. As a result, the metal is apt to agglomerate, especially under reaction conditions and in particular at elevated temperatures. Agglomeration of the catalytically active metal means that the surface area thereof is reduced, which in turn causes a reduction in the catalytic activity of the system and thus, especially in the case of valuable metals, inadequate exploitation of the system""s catalytic capacity.
In patent specification DE 43 24 693.1, the inventor has already suggested over-coming this disadvantage by using metal-fullerene intercalation compounds. These have the advantage of being defined compounds which are not only highly stable on account of their bonding strongly to the catalytically active metals, but can also be reproduced exactly. This is largely due to the fact that direct covalent bonds are formed between the carbon atoms of the fullerene molecule and a metal atom.
One disadvantage of using metal-fullerene intercalation compounds as catalysts is the fact that the supporting material is expensive. In addition, it would be beneficial if the catalytic efficacyxe2x80x94expressed in terms of the content of catalytically active metal or metal compound used in each casexe2x80x94could be enhanced further by making the metal more accessible for the components of the catalysed reaction. The object of this invention is thus to overcome the above-mentioned disadvantages of catalysts with a carbon-based support.
This object is achieved by means of a catalyst which consists of amorphous carbon with molecular planes that have curved surfaces and contain six-membered and non-six-membered carbon rings, optionally having at least one catalytically active, low-valency metal covalently bound thereto.
The invention is based on the surprising discovery that amorphous, carbon with molecular planes exhibiting curved surfaces and containing not only six-membered but also non-six-membered carbon rings are suitable as catalysts and/or as supporting material for catalytically active metals, and have especially desirable properties that result in superior catalysts. Without being bound by theory, it is assumed that this is due to the presence of curved sp2-hybridized carbon layers. The curvature of the layers is not due in this case to simple xe2x80x9crollingxe2x80x9d or xe2x80x9cbendingxe2x80x9d of otherwise intact graphite layers, but is due to the incorporation of non-six-membered rings in the sp2-hybridized carbon layers. The curvature produced thus in the carbon layers is associated with significant differences compared to the geometric and electronic structure of a planar, graphitic sp2-hybridized carbon layer. Besides a considerable stress-induced increase in potential energy, the xcfx80 electrons are not completely delocalized within these curved areas, so that the curved carbon layers can be seen to a certain extent as conjugated double-bond systems.
Thus, the presence of non-six-membered rings in the carbon network not only introduces curvature into the surface of the materials described, but also a modulation of its electronic structure. The combined action of these two effects gives rise to the presence of anchoring sites for metal particles as well as of catalytically active sites.
When used as supporting materials for catalytically active low-valency metals, the carbon material used in the catalysts of the invention, like electron-deficient olefins, are able to form chemical bonds with transition metals in low formal oxidation states. These bonds are chemical bonds formed directly between the metal and the carbon support. They do not, as is the case with conventional carbon supports such as activated charcoal, anchor the metal atoms primarily by way of interactions with terminal heteroatom functionalities, predominantly oxygen.
The amorphous carbon contained in the catalysts of the invention consists of interlacing 6-C rings in which additional rings, mainly 5-C rings, are incorporated. The curved surfaces may be concave or convex. This structure, which is an essential feature of the amorphous carbon used in the catalyst of the invention, can be determined by physical methods, especially x-ray absorption spectroscopy (XAS), as is described by H. Werner et al., in Chem. Phys. Letters 194 (1992), 62-66. The curved areas can also be characterized by their special chemical reactivity; corresponding reaction products have been characterized by IR spectroscopy after partial oxidation of these carbon materials (M. Wohlers, A. Bauer, R. Schlxc3x6gl, Microchim. Acta, submitted 1995, printing). An example of the materials suitable in the context of the invention is the product known as xe2x80x9cKrxc3xa4tschmer sootxe2x80x9d. This is a product which, during the production of fullerenes by the Krxc3xa4tschmer method, is left behind when the fullerenes are separated off. As described in more detail below, the amorphous carbon of the catalyst of the invention may also be obtained by methods other than the one described by Krxc3xa4tschmer.